1. Field of the Invention
The present invention relates to an apparatus for use in a vehicle for controlling the vehicle to travel at a constant speed.
2. Description of the Related Arts
In a conventional constant travel speed control apparatus of a vehicle, the actual speed of a vehicle is detected by a sensor and a target speed is set by a driver. The control apparatus includes a system of one output for one input represented by V(s)/U(s)=b/(s+a)(s is Laplace operator, V(s) is vehicle speed, U(s) is instruction voltage of an actuator, a is unknown characteristic, and b is unknown input distribution characteristic) and a feed-back system based on PID control. Dynamic characteristics include changes in the number of speeds, changes in travel load, namely, changes in road gradient, changes in air resistance, and the weight changes of the vehicle. As a result, the convergence of the actual speed on the target speed is not always constant.
In order to obtain a constant convergence performance of the actual speed on the target speed notwithstanding the dynamic characteristics of the vehicle, it is necessary to adjust a plurality of control gains for calculating an operation amount to be outputted to an actuator for driving a throttle valve according to the change of the number of speeds and the change of travel load. Much labor is required in this adjustment. As disclosed in Japanese Patent Laid-Open Publication No. 1-153344, the operation amount to be outputted to the throttle valve is calculated by (P+1-P+D) control and a travel load is calculated by the difference between a current opening amount of the throttle valve and the opening amount thereof required for a constant travel on a plane ground, and the proportional constant of a forward control system (P+1) and the integration constant thereof are made to be large while the proportional constant of a negative feed-back control system (P+D) and the differential constant thereof are made to be small according to the increase of travel load. Thus, the convergence of the actual speed on the target speed is set at a constant value notwithstanding the difference of the travel load and consequently, a favorable constant speed travel control is achieved.
However, in the conventional apparatus based on the feed-back control by (P+1-P+D) control, the operation required for the object to be controlled is a PTP (point to point) operation for moving the target point (vehicle speed) from a fixed point to a fixed point. That is, an operation guaranteed by the conventional apparatus is only a movement operation from a speed at a certain point in travel to a set target speed and is unsuitable for a CP (continuous path) operation in which the target point moves continuously. That is, the following performance is not guaranteed for a target path speed for converging an actual speed on the target speed and therefore, it is very difficult for the conventional PID control to obtain a constant convergence performance, namely, to converge the actual speed at a constant rate, namely, to converge the actual speed on the target speed along the same target path. Further, in controlling the speed of vehicles of many kinds, a same path cannot be followed by all vehicles due to the difference of engine power. Thus, the conventional apparatus is incapable of generating an appropriate path by the same software.
In addition, the above construction has the following problems. That is, in a vehicle, the input distribution characteristic b of the object to be controlled changes two to three times due to the fluctuation of a travel load. But a control input which operates stably and the actual speed follows the target path favorably by appropriately switching each constant of PID, namely, control gain. However, with the same software, the control gain developed for only one kind of vehicle is incapable of allowing a control output to follow the target path in other kinds of vehicles. It is difficult for the conventional PID control to vary the control gain for other kinds of vehicles and to constitute a robust control system.